half-life 2 is better with ati products
and doom 3 is better with geforce products.
( and you can unlock some more doom3 performance for ati by
unzipping the texture pack from the game directory and changing some values in a specific initialization file )
I would say try and find an X800GT if your can or if someone is selling a 9800XT if would much better than any nVidia card just look at these benchmarks and decide for your self!!